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The GCJ runtime-libraries original source is from GNU Classpath project, but there is a code difference between the libgcj libraries. GCJ 4.3 uses the Eclipse Compiler for Java as a front-end.[5]

In 2007, a lot of work was done to implement support for Java's two graphical APIs in GNU Classpath: AWT and Swing. Software support for AWT is still in development. "Once AWT support is working then Swing support can be considered. There is at least one free-software partial implementations of Swing that may be usable."[6]

The CNI (Compiled Native Interface, previously named 'Cygnus Native Interface') is a software framework for the GCJ that allows Java code to call, and be called, by native applications (programs specific to a hardware and operating-system platform) and libraries written in C++.

CNI closely resembles the JNI (Java Native Interface) framework which comes as a standard with various Java virtual machines.

We use CNI because we think it is a better solution, especially for a Java implementation that is based on the idea that Java is just another programming language that can be implemented using standard compilation techniques. Given that, and the idea that languages implemented using Gcc should be compatible where it makes sense, it follows that the Java calling convention should be as similar as practical to that used for other languages, especially C++, since we can think of Java as a subset of C++. CNI is just a set of helper functions and conventions built on the idea that C++ and Java have the *same* calling convention and object layout; they are binary compatible. (This is a simplification, but close enough.)

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CNI depends on Java classes appearing as C++ classes. For example,[10] given a Java class,